This invention relates to solid state electronic power amplifiers in general and in particular to high fidelity and precision instrumentation amplifiers that require faithful reproduction of analog signals.
Conventional low distortion power amplifiers, designed to deliver relatively large currents to low impedance loads such as loudspeakers or motors, almost universally utilize the push-pull circuit wherein the output stages consist of pairs of tubes or transistors. For minimum distortion in the output signal in push-pull amplifiers the class A mode of operation is used. Unfortunately, when high power push-pull amplifiers are operated in class A, they consume too much current and are inherently expensive; therefore most quality amplifiers operate in the class AB or B mode. These modes of operation can cause distortion in the output signal in the region where one member of the push-pull pair nearly stops conducting, and the other member is just starting to conduct, unless proper operating bias is applied. Vacuum tubes have been and still are used in power stages of quality push-pull amplifiers since they can be properly biased easily with resistors or with low voltage fixed or adjustable power supplies. In solid state amplifiers using power transistors, however, satisfactory biasing methods are more difficult to achieve, creating a distortion problem, which for all of the amplifiers made to this date, has not been satisfactorily solved. This distortion, arising from non-linearities in the output stage because of incorrect biasing, is called crossover distortion; it is detectable in instrumentation circuits and to the trained ear of the audiophile in high fidelity amplifiers.
No matter what biasing scheme is used, it must be such that cut-off does not occur and that an idling current flows through both output devices in the absence of a signal, yet the idling current must not be so high as to cause excessive power dissipation in the output stage. One of the more familiar bias circuits uses temperature feedback from the power stage heat sinks to control devices such as diodes, which regulate the bias point of the power stage. The aim is to establish a good class A operating point for low level output and a smooth transition to class B for high power output. The thermal method, however, has proved to have very slow response, drift, and poor overload recovery.
A more sophisticated method for adjusting the idling current through the output stage was introduced by Karwoski and Suffern. In this method a resistor is inserted into the idling current path and the voltage generated across this resistor is operated on to develop a control voltage proportional to the idling current. This method can suffer from difficulty in properly setting the voltage derived from the series resistor and from variation of the proper control bias point when the level of the signal being amplified changes. Examples of later methods for controlling the bias voltage are found in U.S. Pat. Nos. 4,520,323 and 4,728,903. There is often an optimum level of idling current for which the crossover distortion is a minimum. Deriving a control signal from the output signal for setting the proper point for the idling current, however, requires measuring the idling current in the presence of the large output signal current.